<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        p{
            font-size:30px;
            color:red;
        }
        h3{
            color:blueviolet;
            font-size:30px;
        }
        .myclass{
            color:green;
            font-size:20px;
        }
        

    </style>
</head>
<body>
    <p>hello</p>
    <h3>world</h3>

    <span class="myclass size">类选择器</span>
</body>
</html>